1. Field of the Invention
The present invention relates to a DC-DC converter, and more particularly to a DC-DC converter, in which switching loss and output ripple voltage can be reduced.
2. Description of the Related Art
A DC-DC converter, which converts an input DC voltage into an intended stable DC voltage by switching-control of a semi-conductor device, is highly efficient and can be easily reduced in dimension and weight, and therefore constitutes a vital part in power supplies in various electronic apparatuses, control of electric machinery based on inverter technology, and circuits for lighting various discharge lamps.
FIG. 11 is a circuitry of a conventional step-down DC-DC converter 100. The DC-DC converter 100 includes a field effect transistor Q1, a rectifier diode D3, a choke coil L1, an output capacitor C5, and a control circuit 102, where E is a DC power supply with a voltage Vi, R1 is a resistor as load, and C1 is a junction capacitance between the drain and the source of the field effect transistor Q1.
The DC power supply E has its positive terminal connected to the drain terminal of the field effect transistor Q1 and has its negative terminal grounded. The source terminal of the field transistor Q1 is connected to the cathode terminal of the rectifier diode D3 and to one terminal of the choke coil L1 which has the other terminal thereof connected to one terminal of the output capacitor C5. The other terminal of the output capacitor C5 and the anode terminal of the rectifier diode D3 are grounded. A detection terminal of the control circuit 102 is connected to the other terminal of the choke coil L1, which is the terminal connected to the resistor R1, and an output terminal of the control circuit 102 is connected to the gate terminal of the field effect transistor Q1.
The DC-DC converter 100 operates as follows. Assuming that DC-DC converter 100 is in a steady state condition with the field effect transistor Q1 turned off, when the field effect transistor Q1 is turned on, current flows from the DC power supply E toward the choke coil L1 via the field effect transistor Q1, and a voltage of the choke coil L1 at a side thereof connected to the resistor R1 is smoothed by the output capacitor C5 and applied to the resistor R1. While the field effect transistor Q1 is turned on, energy according to the current is stored in the choke coil L1. And, when the field effect transistor Q1 is turned off, electromotive force is generated at both terminals of the choke coil L1, current maintained by the electromotive force commutates via the rectifier diode D3, and the energy stored is supplied to the resistor R1.
With repetition of the operation described above, a voltage according to a duty ratio [on-time/(on-time and off-time)] of the field effect transistor Q1 is generated at both terminals of the resistor R1. Since the control circuit 102 maintains a constant output voltage independently of variance of the input voltage Vi and the resistor R1, pulse width modulation (PWM) control is performed, where the duty ratio of the field effect transistor Q1 is varied based on the output voltage detected.
In the DC-DC converter 100, at the moment when the field effect transistor Q1 turns on and turns off, a transition period, in which a drain-to-source voltage and a drain current respectively having a non-zero value are present concurrently, appears due to the drain-source junction capacitance C1 and parasitic inductance by wires, whereby switching loss is caused. Since the switching loss is increased due to a higher frequency in performing on-off control, a serious problem is raised if frequency is increased for the purpose of reducing the inductance of a choke coil and the capacitance of an output capacitor to thereby reduce dimension and weight of an apparatus. Also, another problem is that when a reverse bias is applied to the rectifier diode D3 due to the field effect transistor Q1 turned off, a large recovery current is caused to flow through the rectifier diode D3 from the cathode terminal toward the anode terminal at a reverse recovery time, and a large loss is incurred.
What is called “soft-switching technique” is conventionally known, which leverages resonance thereby reducing the switching loss and the loss resulting from the recovery current. For example, a resonant circuitry using junction capacitance of switching and rectifying elements as shown in FIG. 12 is disclosed in order to deal with an extensive variance in output and input voltages (refer to, for example, Japanese Patent Application Laid-Open No. 2003-189602).
Referring to FIG. 12, a DC-DC converter 200 is structured such that the source terminal of a field effect transistor Q1 is connected, via a resonance coil L2, to a connecting portion of a rectifier diode D3 and a choke coil L1, a series circuit consisting of the resonance coil L2 and the rectifier diode D3 is connected in parallel to a series circuit consisting of a clamping capacitor C4 and a field effect transistor Q2, a diode D2 and a capacitor C6 are respectively connected in parallel to the field effect transistor Q2 at the drain and source terminals, a diode D1 and a capacitor C1 are respectively connected in parallel to the field effect transistor Q1 at the drain and source terminals, and a diode 5 is connected in parallel to a series circuit consisting of the field effect transistor Q1 and the resonance coil L2. In the DC-DC converter 200, zero-voltage switching is achieved by resonance caused by the resonance coil L2 and parallel capacitances formed respectively between both terminals of the field effect transistor Q1 and between both terminals of the field effect transistor Q2, and losses and noises can be reduced.
In such a DC-DC converter 200 arranged as shown in FIG. 12, however, when the duty ratio is reduced, sufficient energy cannot be stored in a resonance coil L2, and the voltage at a clamping capacitor C4 is lowered, and if this state goes on, the voltage at the clamping capacitor C4 is eventually reversed thus resulting in prohibiting reset of the resonance coil L2. Also, since the above-described DC-DC converter produces a larger output ripple voltage than the DC-DC converter 100 shown in FIG. 11, the capacitance of an output capacitor must be increased, and/or a low-pass filter must be added, thus inviting a cost increase.